1. Field of the Invention
The present invention relates to a CMOS image sensor, and more particularly, to a method for fabricating a CMOS image sensor to improve a charge capacity in a photodiode.
2. Discussion of the Related Art
Generally, an image sensor is a semiconductor device for converting an optical image into an electric signal. The image sensor can be broadly categorized into a charge coupled device (CCD) and a complementary metal oxide semiconductor (CMOS) image sensor.
In case of the CCD, respective metal-oxide-silicon MOS capacitors are positioned adjacently, wherein electric charge carriers are stored in and transferred to the capacitors. Meanwhile, the CMOS image sensor adopts the CMOS technology of using a control circuit and a signal processing circuit as the peripheral circuit. The CMOS image sensor uses the switching method of sequentially detecting output signals by forming the predetermined number of MOS transistors in correspondence with the number of pixels.
The CCD has the high power consumption and the complicated mask process. Also, it is impossible to provide the signal processing circuit inside the CCD chip, whereby it cannot be formed in one chip. In order to overcome these problems, the sub-micron CMOS fabrication technology has been researched and developed.
The CMOS image sensor may have various types of pixel. Generally, the CMOS image sensor may have a pixel of 3-T (3-Transistor) structure or a pixel of 4-T (4-Transistor) structure. At this time, the pixel of 3-T structure is comprised of one photodiode and three transistors, and the pixel of 4-T structure is comprised of one photodiode and four transistors.
FIG. 1 is a circuit diagram of showing a unit pixel of a 4-T CMOS image sensor according to the related art. As shown in FIG. 1, a unit pixel of a 4-T CMOS image sensor according to the related art is comprised of one photodiode PD and four NMOS transistors Tx, Rx, Dx and Sx. In this case, the photodiode PD functions as the photo-sensing means. The four transistors are formed of a transfer transistor Tx, a reset transistor Rx, a drive transistor Dx and a select transistor Sx. At this time, the transfer transistor Tx transfers optical charges generated in the photodiode PD to a floating sensing node. The reset transistor Rx discharges the optical charges stored in the floating sensing node to detect signals. Also, the drive transistor Dx functions as a source follower, and the select transistor Sx is provided for switching and addressing the optical charges.
In addition, a DC transistor is a load transistor. That is, the DC transistor has a gate electrode, wherein a constant voltage is applied to the gate electrode. Thus, a constant current flows through the DC transistor. Also, ‘VDD’ is a drive power voltage, ‘VSS’ is a ground voltage, and ‘Output’ is an output voltage of a unit pixel.
In the aforementioned CMOS image sensor, the structure of the photodiode PD is very important to the charge capacity.
FIG. 2 shows an operation of a photodiode in a CMOS image sensor according to the related art. In FIG. 2, the reference number ‘1’ indicates a semiconductor substrate, the reference number ‘2’ indicates an STI (Shallow Trench Isolation) layer for isolation of device, the reference number ‘3’ indicates a photodiode ion-implantation diffusion layer, the reference number ‘4’ indicates a depletion layer generated by applying a reverse bias to the photodiode ion-implantation diffusion layer 3, and the reference number ‘5’ indicates an incident light.
In the aforementioned photodiode, the depletion layer 4 is formed, as shown in FIG. 2, when applying the reverse bias to the photodiode ion-implantation diffusion layer 3.
As the light 5 is incident on the depletion layer 4, electron-hole pairs EHP are generated. As shown in the drawings, the holes are discharged to the semiconductor substrate 1, and only the electrons are accumulated to the depletion layer 4, whereby the photodiode operates. Accordingly, as the electron-hole pairs EHP increase in the depletion layer 4, the characteristics of the photodiode improves.
According as the reverse bias is applied to the photodiode ion-implantation diffusion layer 3, the depletion layer 4 has the large inner space in the initial stage. As the electron-hole pairs EHP increase, the holes are discharged to the semiconductor substrate 1, and the electrons are accumulated in the depletion layer 4. With the increase of the accumulated electrons, the inner space of the depletion layer 4 decreases. Thus, the inner space of the depletion layer 4 is decreased little by little so that the inner space of the depletion layer 4 corresponds to the profile of the photodiode ion-implantation diffusion layer 3 before applying the reverse bias.
Accordingly, in case the area of the photodiode ion-implantation diffusion layer 3 is small, the charge capacity is lowered. In other words, if the area of the photodiode ion-implantation diffusion layer 3 is large, the charge capacity is increased.
Hereinafter, a method for fabricating a photodiode of a CMOS image sensor according to the related art will be described with reference to the accompanying drawings.
FIG. 3A and FIG. 3B are cross sectional views of the process for fabricating a photodiode of a CMOS image sensor according to the related art.
In case of a photodiode of a CMOS image sensor according to the related art, as shown in FIG. 3A, an STI (Shallow Trench Isolation) layer 12 is formed in a semiconductor substrate 11. Then, impurity ions for formation of a photodiode are implanted to the semiconductor substrate 11, thereby forming an ion-implantation layer 13.
Subsequently, as shown in FIG. 3B, a thermal process is performed to the ion-implantation layer 13. As a result, a photodiode ion-implantation diffusion layer 14 is formed by diffusing the ions of the ion-implantation layer 13.
The ions of the ion-implantation layer 13, adjacent to the interface with the STI layer 12, may be diffused to the interface between the semiconductor substrate 11 and the STI layer 12 by the thermal process. Also, the ions of the ion-implantation layer 13 may be bonded to the ions of a field channel stop ion-implantation layer (which has the opposite conductive type to the impurity ion for the photodiode), whereby the ions of the ion-implantation layer 13 are extinct. Accordingly, the density in the photodiode ion-implantation diffusion layer 14 adjacent to the STI layer 12 is lower than the density in the central portion of the photodiode ion-implantation diffusion layer 14.
Accordingly, as shown in FIG. 3B, the predetermined portion of the photodiode ion-implantation diffusion layer 14 being adjacent to the STI layer 12 is thinner than the central portion of the photodiode ion-implantation diffusion layer 14.
As explained above, the charge capacity of the photodiode is in proportion to the area of the photodiode ion-implantation diffusion layer 14. In the predetermined portion of the photodiode ion-implantation diffusion layer 14 being adjacent to the STI layer 12, the charge capacity of the photodiode is lowered in proportion to the decreased area of the photodiode ion-implantation diffusion layer 14.
In the dark surrounding of low luminous intensity, when the amount of incident light is small, the small area of the photodiode ion-implantation diffusion layer 14 doesn't cause any problem. However, in the bright surroundings of high luminous intensity, when the amount of incident light is large, the sensing capacity of the CMOS image sensor is lowered since it is impossible to completely accumulate to the electrons changed by the incident light.